Laser spectroscope instruments for measuring trace amounts of moisture in natural gas are discussed in U.S. Pat. Nos. 6,657,198; 7,132,661; 7,339,168; and, 8,547,554. Companies making such instruments include SpectraSensors, Inc. (SSI), the first company to develop a commercial sensor for this application, Ametek, General Electric (GE), and the assignee of this patent application, Advanced Micro Instruments, Inc. (AMI). Tunable diode laser absorption spectroscopy (TDLAS) is the technique used by these instruments for measuring the concentration of certain species such as methane (CH4), water vapor, and many more, in a gaseous mixture. The advantage of TDLAS over other techniques for trace concentration measurement is its ability to achieve very low detection limits and to eliminate interference from absorption by gases other than the gas species of interest.
Apart from concentration, it is also possible to determine the temperature, pressure, velocity and mass flux of the gas under observation. TDLAS is by far the most common laser based absorption technique for quantitative assessments of species in gas phase. A basic TDLAS setup consists of tunable diode laser light source, transmitting (i.e. beam shaping) optics, optically accessible absorbing medium, receiving optics and detector/s. The emission wavelength of the tunable diode laser, viz. vertical-cavity surface-emitting laser, DFB diode laser, etc., is tuned over the characteristic absorption lines of a species in the gas in the path of the laser beam. This causes a reduction of the measured signal intensity, which can be detected by a photodiode, and then used to determine the gas concentration and other properties as described later.
Different diode lasers types are used based on the application and the wavelength of absorption by the gas species of interest. Typical laser types are InGaAsP/InP (in the wavelength range of 900 nm to 1.6 μm), InGaAsP/InAsP (in the wavelength range of 1.6 μm to 2.2 μm), etc. These lasers can be tuned over a narrow wavelength range by either adjusting their temperature or by changing injection current density into the gain medium. While temperature changes allow tuning over 100 cm−1, it is limited by slow tuning rates (a few hertz), due to the thermal inertia of the system. On the other hand, adjusting the injection current can provide tuning at rates as high as ˜10 GHz, but it is restricted to a smaller range (about to 2 cm−1) over which the tuning can be performed. The typical laser line width is of the order of 10-3 cm−1 or smaller. Additional tuning and line width narrowing methods include the use of extra cavity dispersive optics. The basic principle behind the TDLAS technique is simple. The focus here is on a single absorption line in the absorption spectrum of a particular species of interest. To start with, the wavelength of a diode laser is tuned over a particular absorption line of interest and the intensity of the transmitted radiation is measured. The transmitted intensity can be related to the concentration of the species present by the Beer-Lambert law.
Water (H2O) can be measured in natural gas using high-resolution laser absorption spectroscopy where a single vibration-rotation feature of H2O is targeted at a very specific wavelength. As with all molecules that absorb light in the infrared wavelength region, the absorption spectrum consists of many (hundreds to thousands) of individual absorption “lines” within a broad (in terms of wavelength range) “band.” These absorption bands occur throughout the electromagnetic spectrum from the ultraviolet (UV) to the far infrared. For the measurement of H2O in natural gas (consisting mostly of methane CH4), the best wavelength region to use is near 1.9 microns, or 1900 nm. The primary reasons are that lasers are available in this region that are economically viable for an industrial gas sensor, and there is somewhat of a gap in the strong CH4 absorption that is present throughout most of the infrared wavelength region. For water vapor in natural gas, SpectraSensors, Inc. was the first company to develop a commercial sensor for this application starting around the year 2000. Since that time both Ametek and GE have similar systems on the market. All use lasers operating just short of 1900 nm.